The present invention is directed to bicycle derailleurs and, more particularly, to a rear derailleur with a shock absorber to minimize the risk of damage to the derailleur or poor operation of the derailleur when riding over rough terrain.
FIG. 1 shows a typical prior art derailleur (10) such as that shown in U.S. Pat. No. 4,690,663, incorporated herein by reference. That derailleur includes a base member apparatus (14) mounted to a bicycle frame (F); a movable member (18) supporting a chain guide (22) which, in turn, rotatably supports a guide pulley (24) and a tension pulley (28); and links (32,34) pivotably connected between base member apparatus (14) and movable member (18) through 15 pivot pins (38,40,42,44). A control cable (46) has an outer sheath (48) stopped at the base member apparatus (14) through an adjusting barrel (49) and an inner wire (50) connected to link (32) through a connector (54). Pulling and releasing inner wire (50) causes movable member (18) to move relative to base member apparatus (14) which, in turn, causes chain guide (22) to guide a chain (C) to selected positions beneath a plurality of sprockets (G1-G7) under the tension of a return spring (56) in a conventional manner.
FIG. 2 is an exploded view illustrating the detailed structure of base member apparatus (14) and how base member apparatus (14) is mounted to frame (F). As shown in FIG. 2, base member apparatus (14) includes a base member (80) with a base member stopper (82) and a coupling member (84) in the form of a shaft having a tool engaging opening (86) and a threaded portion (88) for screwing into a threaded opening (90) in frame (F). A positioning member (91) in the form of a positioning plate having a positioning nose (92) for facing an abutment (104) on frame (F) and a positioning plate stopper (94) for abutting against base member stopper (82) is rotatably supported on coupling member (84). A coil spring (96) having a first end (98) retained in an opening (100) in positioning member (91) and a second end (102) retained in an opening (not shown) in base member (80) is also supported around coupling member (84). An adjusting screw (108) is screwed into a threaded opening (110) in positioning nose (92) so that turning adjusting screw (108) adjusts the position of positioning nose (92) relative to abutment (104) for adjusting the twist angle of spring (96).
Other derailleurs may be constructed in accordance with the teachings of U.S. Pat. No. 4,610,644, incorporated herein by reference wherein the adjusting screw (108) is omitted, in accordance with U.S. Pat. No. 4,235,118, wherein the coil spring (96) is omitted, or in many other ways. In any event, when the bicycle is ridden on very rough terrain, and especially downhill on such terrain where the bicycle may be subjected to severe shocks, the chain (C) sometimes violently oscillates up and down. This violent oscillatory motion is transmitted to the base member assembly (14)through the chain guide (22) and links (32,34), thus resulting in a hammering effect between positioning plate stopper (94) and base member stopper (82) or between adjusting screw (108) and abutment (104). Over time, the hammering motion can destroy the base member (80) or positioning member (91).